Unit 7

Question 1

spontaneous change

Answer 1

is one that
occurs without a continuous input
of energy from outside the system
(although activation energy may be
required to initiate it)
-If a change is spontaneous in one direction, it will be non-
spontaneous in the reverse direction.
- Note that non-spontaneous reactions and processes may be
driven with the continual input of energy.

Question 2

Entropy

Answer 2

Positive value of DS indicates increased dispersal of energy.
Negative value of DS indicates decreased dispersal of energy.
* Some think of entropy as “disorder” or “randomness.” (Pure
thermodynamicists don’t like this characterization, but it captures the main idea.)
* Entropy is a state function, and is extensive.
* S° = standard molar entropy: entropy of 1 mole of the pure
substance in its standard state (pure substance in its most stable form for
solids and liquids, 1 M for solutions, 1 atm pressure for gasses and a specified
temperature, usually 25°C)
-Entropy increases with chemical complexity & flexibility
-As temperature increases, entropy increases.

Question 3

lower entropy

Answer 3

fewer molecules –
energy is less disperse

Question 4

higher entropy

Answer 4

more molecules –
energy is more disperse

Question 5

high-entropy configurations

Answer 5

The answer, in short, is that high-entropy configurations
(reflecting large freedom of motion and the dispersal of energy)
can be achieved in more ways than low-entropy (more highly
ordered) configurations. High-entropy configurations are
therefore more likely to occur

Question 6

higher probability outcome (macroscopic state):

Answer 6

more possible microstates = higher S

Question 7

lower probability outcome (macroscopic state):

Answer 7

fewer possible microstates = lower S

Question 8

Boltzmann’s equation

Answer 8

formalizes the relationship between the
number of possible microstates and the entropy of a system

Question 9

Entropy is a measure of energy dispersal

Answer 9

Absolute entropies can be calculated from the number of
microstates (W) a system may occupy
Entropy increases with
* number of molecules (especially gas molecules)
* molecular motion (temperature, # bonds, flexibility…)
Spontaneity seems to be promoted by an increase in entropy

Question 10

2nd law thermodynamics

Answer 10

Spontaneous reactions proceed in the direction that increases the
entropy of the universe (system plus surroundings)
-Thus, any decrease in the entropy of the system must be offset by
a larger increase in the entropy of the surroundings for that
process to be spontaneous
The 2nd law has many profound implications. Among these are:
* Isolated systems always evolve toward higher-entropy (more energy-
disperse) states.
* The entropy of the universe is always increasing.
* Entropy is the “arrow of time” – this law gives time a direction.
* Eventual “heat death” of the universe? (Equal distribution of energy)

Question 11

2nd Law Links Entropy, Heat Flow and Temperature

Answer 11

A key part of the preceding idea that flows out of the 2nd law:
* Temperature, heat flow and entropy are linked:
* If heat flows into a system from the
surroundings, the entropy of the system
increases. The surroundings lose entropy.
* And visa versa, for heat flow out of a warmer system into colder surr.
* The amount by which a given amount of heat flow changes entropy
depends on temperature (T). If T is very low, the effect on entropy
(think # of accessible microstates) can be enormous. At higher T, the
change in entropy for a given flow of heat is less.
* In fact, for (ideal fictive) reversible heat flow, which happens in such tiny
increments that the system remains at equilibrium throughout the
transition

Question 12

3rd law thermodynamics

Answer 12

A perfect crystal has zero entropy at absolute zero.
-A “perfect” crystal has flawless alignment of all
its particles. At absolute zero, the particles have
minimum energy, so there is only one
microstate.
-This enables us to find the absolute entropy (S) of a
substance at a given temperature.
-In principle this can be done by cooling the substance
to as close to 0 K as possible, then heating it in tiny
increments, measuring q and T to calculate DS for
each increment. Finally, the entropy change from
each tiny step is summed up to determine S.

Question 13

Standard Molar Entropy,

Answer 13

The entropy of 1 mole of the
substance under standard conditions: (pure substance in its most
stable form for solids and liquids, 1 M for solutions, 1 atm pressure for
gasses and a specified temperature, usually 25°C.)
– Standard molar entropies are absolute entropies measured against an
absolute reference point (= perfect order)

Question 14

The standard entropy of reaction (∆S°rxn)

Answer 14

is the entropy change
that occurs when all reactants and products are in their standard
states. Because entropy is a state function and is extensive,
calculating entropies of reaction is straightforward

Question 15

Second Law of Thermodynamics:

Answer 15

Spontaneous reactions
proceed in the direction that increases the entropy of the universe
(system plus surroundings).

Question 16

Spontaneous process:

Answer 16

favored by decrease in enthalpy (-DHrxn)
–favored by increase in entropy (+DSrxn)

Question 17

Gibbs Free Energy Change

Answer 17

evaluates spontaneity as a function
of enthalpy and entropy of the system alone
Bottom lines:
* Lowering free energy is the driving force of chemical reactions.
* Negative DHsys and positive DSsys favour spontaneity.
* Entropic contribution to free energy change (-TDS) is
increasingly important at higher temperatures.

Question 18

Free Energy Change and the Work a System Can Do

Answer 18

• The magnitude of DG is important – we will see next class that this tells us just
  how spontaneous a process is.
• It turns out that DG is the maximum useful work that can be done by a system
  as it undergoes a spontaneous process at constant temperature and pressure:
• DG is also the minimum work that must be done on a system to drive the
  occurrence of a non-spontaneous process. (More about driving non-spontaneous
  process in the last part of the notes for chapter 20)
• Awareness of this relationship between DG and work will be useful in our
  discussion of electrochemistry (the last subject of CHM135 lectures).
• Why do I specify “maximum” and “minimum”?
  – Inevitably, in real processes some energy is lost (not captured as work, but instead is
  dissipated through the irreversible entropy increase of the universe - usually as heat that
  escapes out to the wider world). A detailed explanation involves the requirement for
  reversibility (ie, DSuniv = 0) and is discussed further in advanced thermodynamics courses

Question 19

Standard Free Energy Change

Answer 19

Free energy change (DG) is a state function and is extensive, just like its
component thermodynamic variables, DH and S (and DS). Thus, DG can be
calculated in a straightforward way from related reactions, including:
* DG scales linearly with amount (extensive property)
* the sign of DG changes when the reverse reaction is considered (state function)
* DG reported depends on the states of reactants and products (e.g. liquid vs. gas)
(see also Thermo 1 packet slide 45, in which analogous calculations for DH were described)
* The standard free energy change (∆G° or ∆G°rxn) is defined the same way
standard enthalpy change and standard entropies change are defined. It is the
Gibbs free energy change that occurs when all reactants and products are in
their standard states.
* One way to calculate ∆G° is by combining standard enthalpies
and entropies of reaction

Question 20

Standard Free Energies of Formation

Answer 20

Also along familiar lines, we define ∆G ̊ƒ as the standard free energy of
formation of a compound from its elements in their standard states.
(Just as we defined and used DH°f, see Thermo 1 packet slides 77-81)
* ∆G ̊ƒ values are tabulated in many places, including Appendix B of our text.
* ∆G ̊ƒ have similar properties to ∆H ̊ƒ , including
* ∆G ̊ƒ of an element in its standard state is ZERO
* This gives us another way to calculate standard free energies of reaction using
tabulated data:
∆G ̊rxn = ∆G ̊ƒ (products) – ∆G ̊ƒ (reactants)

Question 21

Thermodynamics vs. Kinetics

Answer 21

Because of activation barriers, spontaneous reactions can be slow
-ΔG tells us whether a reaction will/won’t proceed (thermodynamics).
The free energy of activation (which includes Ea) tells us how fast a
reaction proceeds (kinetics).

Question 22

How do we make a non-spontaneous reactions happen?

Answer 22

It must be driven by coupling the non-spontaneous reaction
with a spontaneous reaction of sufficiently favorable DG

Question 23

Adenosine triphosphate (ATP) is a “high energy” molecule:

Answer 23

potential energy stored in this molecule can be retrieved by its hydrolysis to
form adenosine diphosphate (ADP), hydrogen phosphate and hydronium ions

Question 24

Magnitude of G

Answer 24

tells us how far out of equilibrium our mixture is
-If Q and K are very different, DG has a very large value (negative or positive). The reaction
releases or absorbs a large amount of free energy as it proceeds to equilibrium.
If Q and K are nearly the same, DG has a very small value (negative or positive). The
reaction releases or absorbs very little free energy as it proceeds to equilibrium

Question 25

G

Answer 25

∆G (free energy change under the present conditions) depends on how
much product and reactant is present at that instant (Q) compared to
their equilibrium values (K), the temperature and the gas constant